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Wednesday, May 21, 2008

Homology, Darwinism, and Logic

[This is a section from one of my chapters on Darwinism and Intelligent Design.]

One argument from homology to Darwinism is philosophical and does not rely on any empirical factors. The objection is made that a conscious designer would never use similar structures in different organisms to accomplish different tasks. Therefore, the random process of natural selection is the better explanation.

The empirical evidence for homology is very questionable (see Jon Wells, Icons of Evolution and M. Denton, Evolution: A Theory in Crisis), but the logic behind the philosophical argument is flawed as well. Why, pray tell, should a designer employ entirely different structures for different purposes—say for a wing and for a hand (which have some structural bone similarities)—when similar structures accomplish various goals quite well? How can the Darwinists read the mind of the nonexistent (or at least nondesigning) God? There seems to be no moral or logical principle at hand to invoke against such a designer. Moreover, many human designers employ similar structures for divergent purposes. If this is the case for human designers, why not so for a nonhuman designer of the structures of living things themselves?

14 comments:

Sure makes sense to me. And from a theological and aesthetic standpoint, certain forms which resemble each other to greater and lesser degrees may have purposes utterly alien to their physiological or survival function.

Why should a platypus be "duck-billed"?

And why shouldn't two eyes on the front of a face seem "more human" to us?

If, for example, varying types and degrees of resemblance to humans and human projection onto animals were key reasons for a diversity of animal forms . . . so that we would "see ourselves in" creatures in different ways and relate to them--and to God, and He to us--among them. . . .

Wouldn't function-irrelevant homology be some part of that aesthetics of Creation?

"Moreover, many human designers employ similar structures for divergent purposes."

They may indeed, but they don't have to. Evolution has to cobble together novel structures from existing structures. Are you suggesting the "non-human" designer is constrained by this when human designers aren't. Unless you are proposing an unending sequence of design events this idea does not jibe with certain aspects of the fossil record. What are we to make of the apparent whale sequence found here? Are the intermediate forms just practice?

The arguments that you make here againsthomology are pretty outdated. You needto study up on the modern view of developmentalevolutionary biology. There books bySean B. Carroll are a good start.

Homology of structures between species isgoverned by genes called "hox" genes. Thedifferences in structures occur throughduplication of hox genes, and through thechanges in timing of the action of these genes.

"I question the sufficiency of natural selection and mutation to account for everything in the biosphere."

You're not alone. I believe there are other mechanisms such as genetic drift that may play an important role. But there is no evidence that anything beyond natural processes are required.

"Some development within species is compatible with my view, but not macro-evolution according only to natural, unintellignet causes. The whale case is much disputed, in any case."

You can always dispute direct ancestry. I doubt if that case is provable and if you are looking for certainty you won't find it in paleontology. The pattern and timing of the distribution is what is important here. Whether direct ancestors or kissing cousins what other process other than descent with modification could be at play?

Since the species that exist now didn't exist for the most part 60, 100, or 500 million years ago why would you think that there is some sort of dividing line beyond which natural evolutionary processes can't cross. Again, is the alternative innumerable multiple creations or some sort of continual interference by an undected agent?

I find the attempt to decide exactly where microevolution and macroevolution differ less interesting than the question of whether descent with modification from a high-genetic-potential population of a few distinct kinds, or descent with modification from zero genetic potential into some nonzero amount, makes more sense.

I have no problem with natural selection, up to the point where natural selection ceases to be the uncontrolled version of breeding efforts and becomes the hero of Darwin's mythopoeic works--and his grandson's, too.

(Charles Darwin was the grandson of radical, naturalist, and bad poet Erasmus Darwin, who wrote to long verse narratives attempting to argue for the theory his grandson later wrote a prose epic about.)

You avoided my philosophical point. I'll look into hox genes, but I doubt they will explain everything adequately.

... the philosophical point...One argument from homology to Darwinism is philosophical and does not rely on any empirical factors. The objection is made that a conscious designer would never use similar structures in different organisms to accomplish different tasks. Therefore, the random process of natural selection is the better explanation.

That simply is a strawman that you are demolishing. First of all,there isn't any such thing as "Darwinism", at least not in thiscentury. Second, "arguments frombad design" are usually used against creationists as a responseto claims by creationists that"design is obvious".

Such discussions really don't occurin the scientific community, becauseyou would have to have an actualscientific theory of manufacturedbiology to propose such hypotheses,and such a thing does not yet exist.

You beg the question again. If certain species exist now and did not, say, 50 million years ago, that does not prove that they came about through only natural, unintelligent causes. It simply means they did not exist then, if that is true at all. There are genetic limits to species change. Mutations are usually harmful, not helpful.

I am arguing for design as an explanatory category, not for innumerable special creations all over the place. That is a straw man. There may be considerable change within life forms over time, but within certain forms.

Does "undected" agent mean "undetected agent"? If so, the agent is detected through the signs of intelligences: the irreducible complexity of aspects of the cell, the fine tuning of the unverse, the specified complexity of information in DNA, etc.

"You beg the question again. If certain species exist now and did not, say, 50 million years ago, that does not prove that they came about through only natural, unintelligent causes."

The point is that species or "kinds" or whatever you want to call the classification is not static.

"There are genetic limits to species change."

How did you determine this and what are they?

"Mutations are usually harmful, not helpful."

They are usually neutral. Neutral mutations may very well supply some of the material for future genetic change.

"I am arguing for design as an explanatory category, not for innumerable special creations all over the place. That is a straw man. There may be considerable change within life forms over time, but within certain forms.

I don't think so. I'm asking for a coherent alternative to descent with modification. If say, 63 million years ago, all placental mammals were shrew-like animals (which fits the fossil evidence, how can you say that change is limited to certain "forms" (whatever that means) and still explain the diversity of placental mammals today?

"Does "undected" agent mean "undetected agent"?

Yep. I'll try to keep it in English.

"If so, the agent is detected through the signs of intelligences:"

And this is determined through a weak analogy to human design. No process, no timeline - no dice. It is a less than compelling alternative to evolutionary theory. If fact, you cannot even compare them on equal terms.

1. Is Stephen Jay Gould a straw man? He used this argument, as do other Darwinists in their case agasnst design.

2. Of course, Darwinism still exists! Other mechanisms besides natural selection and mutation have been added, but those are nonnegotiable and Darwinian. It is called the newo-Darwinian synthesis.

What other mechanism? Basically theclaim of design has no mechanism.Not merely that, it is the claimof *no possible mechanism*.

The term Darwinian synthesis refersto the notion of studying "descentwith modification and natural selection"in the context of genetics.

Actual, "Darwinism" which is to saythe strict adherence to the principlesoutlined by Darwin has been superscededby a more modern view. Consequently,there aren't any more Darwinists.

3. Supoptimal design does not equal no design. That is really a bad argument.

You just don't get it. The claim ofdesign has no theory that will tell uswhat to expect in terms of "design".

The creationist argues that the structure is evidence of "good design",yet he or she cannot refute the argumentthat the same structure is evidence of"bad design". Hence, apparently designsare all simultaneously both good andbad, rendering the concept of designimpotent in biology.

You raise a good question, Doug: Why should God strive for good design, when merely adequate design will work? God's economics don't work the same as human economics -- quality is probably not a function God needs to achieve in order to get the most design for his buck.

But I fear that quickly decays as an argument. Is God arbitrary and capricious? Really? Sez who?

I saw an interesting video in which Dawkins stated that flatfish are evidence against design because "no one would ever design a fish like this", which of course really means, "I wouldn't have designed it like this, so therefore it cannot have been designed."

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Nothing on this blog represents the position of Denver Seminary. I am a Christian, philosopher, teacher, writer, and preacher, who is Professor of Philosophy at Denver Seminary. My most recent of my twelve books is Philosophy in Seven Sentences. My magnum opus is Christian Apologetics: A Comprehensive Case for Biblical Faith (InterVarsity Press, 2011). I have published ten others, including Truth Decay and On Jesus. I direct the Christian Apologetics and Ethics MA program at Denver Seminary.